In order to ensure the operational reliability of drive units, such as in particular internal combustion engines, which are supplied with liquid fuels, it is conventional practice to provide fuel supply systems with water-separating fuel filters. The separation of water which is contained or entrained fuel and in particular in diesel fuel is required to protect the sensitive injection system from damage. The water which has been separated in these systems is loaded with various types of organic matter, for example, with suspended droplets of oil from the diesel fuel flowing through the fuel filter. Therefore, for reasons of environmental protection, the release of water which has been separated from the fuel filter systems into the environment without further treatment is not an option.